Dimple-forming burnishing tool and dimple-forming burnishing method

ABSTRACT

A burnishing tool includes a mandrel; and a frame fitted to the mandrel and holding balls contacting an outer peripheral surface on the mandrel. The outer peripheral surface has a contact surface with the balls. The contact surface is at a side of the outer peripheral surface which is closer to the work surface. The contact surface and the work surface have a distance therebetween in a direction normal to the work surface. A distance at a side of the mandrel closer to the processing machine and a distance at the opposite side of the mandrel have a distance difference therebetween. A diameter of the outer peripheral surface on the mandrel is set in such a way that the distance difference with the mandrel flexed during formation of the dimples becomes smaller than that before flexing of the mandrel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2012-035752 filed on Feb. 22, 2012; theentire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a dimple-forming burnishing tool and adimple-forming burnishing method for forming a dimple on a work surfaceby contacting a ball with the work surface.

2. Description of Related Art

There is a technique of forming fine dimples for keeping a lubricant ona sliding surface of a sliding member such as a bearing to reducefriction resistance of the sliding surface. The surface treatment forforming the dimples on a work surface such as the sliding surface hasbeen known as a dimple-forming by a WPC treatment (fine-particleshot-peening), a dimple-forming by a laser-machining and the like.

The dimple-forming by the WPC treatment is a surface treatment ofshooting fine particles, smaller than those for an ordinary peening, ata high speed to be struck on the work surface for forming dimples. Thedimple-forming by the laser-machining is a surface treatment ofradiating laser beam on the work surface using an exclusive processingmachine for forming dimples.

There is proposed another surface treatment for forming dimples on awork surface using a burnishing tool (see, for example, Patent Document1). The burnishing tool includes a cylindrical frame fitted to a mandrelfrom the outside. The cylindrical frame holds rotatably rolling elementsto come radially out of or into the outer peripheral surface of theframe. With the mandrel fitted to a processing machine rotated, theprojections, formed on the outer peripheral surface of the mandrel,engage rolling elements such as balls. The rolling elements vibratinglyrotate on the inner peripheral surface of a work as a work surface.

-   [Patent Document 1] Japanese Patent Publication NO. 4575899

However, when the surface treatment of forming dimples is applied to thework surface of a narrow portion such as the side surface of the grooveportion of a compressor component, the problem has been proposed asfollows.

That is, the WPC treatment requires shooting particles at every surfacetreatment, which raises machining cost, and is not efficient. Thelaser-machining is difficult to be applied to the work surface of thenarrow portion such as a side surface of the groove portion.

The burnishing tool as described in Patent Document 1 is a tool used forthe surface treatment to the inner peripheral surface of a hole formedto a work, and is not applied to the surface treatment to a work surfacesuch as a side surface of the groove portion. If the tool is used on thework surface of the narrow portion such as the side surface of thegroove portion, a tool diameter becomes smaller, and the tool is flexedduring formation of dimples. This causes difficulty in to formingdimples by secure contacting all of the rolling elements on the worksurface.

SUMMARY OF THE INVENTION

The invention is directed to a dimple-forming burnishing tool and adimple-forming burnishing method which efficiently and securely formdimples to a work surface of a narrow portion such as a side surface ofa groove portion.

A first aspect of the invention provides a dimple-forming burnishingtool. The dimple-forming burnishing tool includes a mandrel having anend side fitted and fixed to a processing machine for rotating; and aframe fitted to another side of the mandrel from the outside, andholding balls contacting an outer peripheral surface of the mandrel. Theballs on the outer peripheral surface contact a work surface with themandrel and the frame rotated for forming dimples on the work surface.The outer peripheral surface has a contact surface with the balls. Thecontact surface is at a side of the outer peripheral surface which iscloser to the work surface. The contact surface and the work surfacehave a distance therebetween in a direction normal to the work surface.A distance at a side of the mandrel closer to the processing machine anda distance at the opposite side of the mandrel to said side have adistance difference therebetween. A diameter of the outer peripheralsurface on the mandrel is set in such a way that the distance differencewith the mandrel flexed during formation of the dimples becomes smallerthan that before flexing of the mandrel.

According to the invention, if the mandrel is flexed during formation ofthe dimples, the burnishing tool of a smaller diameter enables all ofthe balls arranged on the outer peripheral surface to be pressed againstthe work surface with the mandrel flexed. This operation forms thedimples of approximately a uniform depth at a predetermined whole regionon the work surface of a narrow portion which is a side surface of, forexample, a groove portion.

That is, the dimples are efficiently and securely formed to the worksurface of the narrow portion such as a side surface of the grooveportion.

Therefore, the work surface is formed with very fine and uniformdimples, and keeps a lubricant in the dimples. This prevents a lubricantfrom running out, and reduces frictional resistance, thereby enhancingthe sliding property.

The dimple-forming burnishing tool is fitted to a generic processingmachine (machine tool) such as a machining center, or an NC millingmachine for use, and is capable of forming dimples in a shape which isrequired with ease and at an inexpensive cost.

The mandrel has a first sectional line which is positioned on the outerperipheral surface and at said side of the mandrel closer to the worksurface. The first sectional line is defined by sectioning the mandrelby a plane which is normal to the work surface and includes an axis ofthe mandrel. The work surface has a second sectional line on the worksurface. The second sectional line is defined by sectioning the worksurface by the plane. The first sectional line and the second sectionalline have a parallel degree therebetween. A diameter of the outerperipheral on the mandrel at the opposite side to a side closer to theprocessing machine is set to be larger than that at said side close tothe processing machine, so that the parallel degree with the mandrelflexed during formation of the dimples becomes smaller than that beforeflexing of the mandrel.

According to the invention, it is applicable to the case where the worksurface is a plane. With the mandrel is flexed during formation of thedimples, the sectional line at the side of the outer peripheral surfacecloser to the work surface is approximately parallel with the sectionalline on the work surface. This permits all of the balls arranged on theouter peripheral surface to be pressed against the work surface at afurther uniform strength.

The outer peripheral surface of the mandrel has a side shape in afrustum of a circular cone which diverges toward the opposite side tosaid side closer to the processing machine.

According to the invention, with the mandrel being flexed, the sectionalline at a side of the outer peripheral surface closer to the worksurface is exactly a curved line, not a straight line. On the otherhand, a parallel degree between the sectional line at a side of theouter peripheral surface closer to the work surface and the sectionalline on the work surface become sufficiently smaller. Herein, formationof the outer peripheral surface of the mandrel in a side-shape of afrustum of a circular is made by minor change from formation of themandrel in a side-shape of a straight circular cylinder in themanufacturing process of the mandrel itself.

The balls are spaced apart from each other in a peripheral direction ofthe frame and in a direction parallel with the axis of the frame.

According to the invention, it is enabled to further efficiently formthe dimples on the work surface.

The frame includes: a frame body fitted to the mandrel from the outsideand defining first through-holes for the balls to be fitted thereto,respectively; and a frame cover fitted to the frame body from theoutside, having the balls projecting radially outward from the outerperipheral surface of the frame, and defining second through-holes whichcoincide in position with the first through-holes for preventing theballs from dropping out of the first through-holes, respectively.

According to the invention, it facilitates machining such as formationof holes for holding the balls, thereby enhancing manufacturingoperability for the dimple-forming burnishing tool.

A second aspect of the invention provides a dimple-forming burnishingmethod which includes contacting the balls on the work surface to formdimples on the work surface using the dimple-forming burnishing toolaccording to the first aspect of the invention.

According to the invention, it is capable of serving as the sameoperation and benefits as those of the first aspect of the invention.

Benefits of the Invention

According to the invention, this enables the dimples to be efficientlyand securely formed on the work surface of the narrow portion such as aside surface of the groove portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a half sectional view which shows a whole structure of adimple-forming burnishing tool according to an embodiment of theinvention.

FIG. 2 is a general perspective view of the burnishing tool as show inFIG. 1, which shows an example of working condition.

FIG. 3 is a schematic view of the front periphery of a mandrel beforethe mandrel is flexed.

FIG. 4 is a schematic view of the front periphery of the mandrel withthe mandrel flexed during formation of dimples.

FIG. 5 is a perspective view of the mandrel which schematically showsthe front periphery of the mandrel with the mandrel flexed duringformation of dimples.

FIGS. 6A and 6B are views which show the state of dimple-forming by aburnishing tool, FIG. 6A is a general view as viewed from the positionopposed to the work surface, and FIG. 6B is an enlarged view of thedimples formed on the work surface as shown in FIG. 6A.

FIGS. 7A, 7B and 7C are schematic views of a mandrel which is pressedinto the work surface with the pressing-into amount varied, FIG. 7Ashows a case of a smaller pressing-into amount, FIG. 7B shows a case ofan appropriate pressing-into amount, and FIG. 7C shows a case of alarger pressing-into amount.

In the Figures, the numeral F1 indicates a front direction in alongitudinal direction of the burnishing tool, and the numeral R1indicates a rear direction in a longitudinal direction of the burnishingtool.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, an embodiment according to the invention will be described withreference to the drawings. FIG. 1 shows a half-sectional view of adimple-forming burnishing tool (hereafter, referred to as a burnishingtool) according to the embodiment of the invention. In FIG. 1, the upperside of the burnishing tool relative to the axis CL is shown by thesectional view, and the lower side thereof is shown by the side view. Itis noted that the front direction F1 and the rear direction R1 of theburnishing tool are set for clear description, as shown in FIG. 1.

As shown in FIG. 1, the burnishing tool 1 includes: a mandrel 2 having arear end side which is rotatably fitted to a processing machine (notshown in Figs.) such as a machining center or an NC milling machine; anda frame 4 which is fixed to the front end side of the mandrel from theoutside and which holds balls 3 contacting the outer peripheral surface23 on the mandrel 2.

The mandrel 2 is formed in a round bar-shape as a whole. The mandrel 2includes a shank portion 21 fitted to the processing machine, and a bodyportion 22 having the frame 4 fitted thereto from the outside. Themandrel 2 is composed of an integral member having the shank portion 21and the body portion 22 formed integrally with each other, therebypreventing additional vibrations from transmitting, enhancing stiffness,and ensuring stable flexibility.

The shank portion 21 adopts a straight shape as shown in FIG. 1, and, inaddition, various shapes such as a taper shape for adapting to aprocessing machine for fitting. The body portion 22 includes a trunkportion 24 continuous with the shank portion 21; and a front portion 25which is smaller in diameter than the trunk portion 24 and which has anouter peripheral surface 23 contacting the balls 23. The front portion25 may not be necessarily smaller in diameter than the trunk portion 24.

The frame 4 is formed in approximately a cylindrical shape as a whole.The frame 4 includes a frame body 41 which is fitted to the mandrel 2from the outside, and a frame cover 42 which is fitted to the frame body41 from the outside.

The frame body 41 includes a holding portion 44 which is formed withfirst through-holes 43 for the balls 33 to be fitted therein. The framebody 41 includes a fitting portion 45 which is larger in diameter thanthe holding portion 44 for fitting the frame 4 to the mandrel 2. Theframe body 41 includes an intermediate portion 46 which connects theholding portion 44 and the fitting portion 45. Each of the firstthrough-holes 43 has an inner diameter slightly larger than the diameterof each ball 3. The respective balls 3 are fitted into the correspondingfirst through-holes 43 for holding.

The fitting portion 45 of the frame body 41 has, on a rear side thereof,thread-holes 47 (for example, three pieces) which are formed at equalangular intervals on the circular periphery and which extend radially ofthe frame body 41. The mandrel 2 has the trunk portion 24 which isformed, on the outer peripheral surface, with an annular recessedportion 26 in a V-shaped section. Stopper screws 61 are screwed into thethread-holes 47 to be pressed against the inner surface on the annularrecessed portion 26 formed to the trunk portion 24, which fixes theframe 4 to the mandrel 2. Herein, the thread-holes 47 and the annularrecessed portion 26 are located in the longitudinal direction in such away that the distal ends of the stopper screws 61 come in contact withthe front oblique surface of the inner surface on the annular recessedportion 26. Therefore, the rear end surface on the intermediate portion46 of the frame body 41 comes in contact with the front end surface onthe trunk portion 24 of the mandrel 2, which securely positioned theframe 4 to the mandrel 2.

The frame body 41 of the frame 4 has a stepped shape in which theholding portion 44 and the fitting portion 45 larger in diameter thanthe holder portion 44 are connected by the intermediate portion 46, andis fixed to the mandrel 2 at the fitting potion 45 of a larger diameter.This constitution enhances the stiffness compared with a frame in acylindrical shape having little step, and is capable of rigidly fixingthe frame 4 to the mandrel 2. This prevents the distal end side (frontside) of the burnishing tool 1 from being displaced, and therebyachieves further stability.

The frame cover 42 in a cylindrical shape is formed with secondthrough-holes 48 which coincide in position with the first through-holes43, respectively. Each of the second through-holes 48 has an innerdiameter smaller than the diameter of each ball 3. The secondthrough-holes 48 allow the balls 3 to project radially outward from theouter peripheral surface of the frame 4, and have function to preventthe balls 3 from dropping out of the first through-holes 43.

The frame cover 42 is fitted to the frame body 41 from the outside withthe first through-holes 43 and the second through-holes 48 coinciding inposition with each other, and is fixed to the frame body 41 by clamps 5(for example, two pieces). Each of the clamps 5 in an L-shape includes afitting portion 51 having a through-hole 53 for a screw member 62 to beinserted therein, and a holding portion 52 having the frame cover 42held between the holding portion 52 and the frame body 41. The framebody 41 has the fitting portion 45 which has, at the front side,threaded holes 49 (for example, two pieces) formed at equal angularintervals on the circular periphery and extending radially of the framebody 41. The screw member 62 is inserted through the through-hole 53 tobe screwed into the threaded hole 49, and thereby the frame cover 42 isheld and fixed between the clamp 5 and the frame body 41.

The frame 4 is constituted by two separate members of the frame body 41and the frame cover 42. This constitution facilitates machining forforming holes for holding the balls 3 held, and enhances manufacturingoperability for the burnishing tool 1. It is noted that the frame 4 maybe constituted by an integral member having the frame body 41 and theframe cover 42 formed integrally with each other.

The balls 3 are arranged at angular intervals in the peripheraldirection of the frame 4, and are arranged at intervals in a directionparallel with the axis CL (referred to as an axial direction) of theframe (common axis with that of the burnishing tool 1). Thisconstitution enables dimples 71 (see FIG. 4) to be more efficient formedon the work surface 7 (see FIG. 2). Herein, the positions of the firstthrough-holes 43 in the frame body 41 and the positions of the secondthrough-holes 48 in the frame cover 42 are determined, depending on thearrangement layout of the balls 3.

In FIG. 1, the balls 3 are axially arranged in ball rows at therespective four peripheral positions which are spaced at intervals of 90degree in the peripheral direction of the frame 4. The balls 3 inneighboring ball rows are slightly displaced axially from each other. Itis noted that the placement number of the balls 3 and the arrangementlayout of the balls 3 such as peripheral or axial arrangement intervalsof the balls 3 may vary, depending on a size or a material of the worksurface 7 (see FIG. 2).

FIG. 2 is a general perspective view of the burnishing tool which showsan example of the use. FIG. 3 is a schematic perspective view of thefront periphery of the mandrel before the mandrel is flexed. FIG. 4 is aschematic view of a front periphery of the mandrel with the mandrelflexed during the formation of dimples. FIG. 5 is a perspective viewwhich schematically shows the front periphery of the mandrel with themandrel flexed during the formation of dimples. In FIGS. 3 and 4, theshape of the mandrel 2 is exaggeratingly drawn for convenientdescription, and the number of balls 3 is drawn differently from theactual one (FIG. 7 is the same as ones). In FIG. 5, the shape of themandrel 2 is exaggeratingly drawn for convenient description, and theballs 3 are not shown in the Figure.

As shown in FIG. 2, the burnishing tool 1 is a tool for contacting theballs 3 on the outer peripheral surface 23 (see FIG. 1) with the worksurface 7 on the work W with the mandrel 2 and the frame 4 rotating,thereby to form the dimples 71 (see FIG. 4) on the work surface 7. Thework surface 7 is a side surface on a groove portion S formed to thework W.

As shown in FIGS. 3 to 5, the burnishing tool 1 is pressed into the worksurface 7 by a predetermined press-into amount in a direction of P (seeFIG. 2) from the position (see FIG. 3) where the ball 3 at the mostdistal end side (front side F1) contacts just or slightly the worksurface 7 (see FIG. 4). At this time, the mandrel 2 flexes oppositely tothe work surface 7 as it extends toward the distal end side, with theaxis CL (common axis with that of the burnishing tool 1) having amaximum flexion of δ at the distal end of the mandrel 2, thereby formingthe dimples 71 on the work surface 7.

In the embodiment, with the mandrel 2 flexed during formation of thedimples 71 (see FIGS. 4 and 5), the diameter of the outer peripheral 23on the mandrel 2 is set so as to vary along the axis CL depending on theshape of the work surface 7. Therefore, in relation to the distance Lbetween the contact surface 23 a of the outer peripheral surface 23 on aside closer to the work surface 7 and the work surface 7 and normal tothe work surface 7, the distance difference between the side closer tothe processing machine (processing machine side or rear side) and theopposite side (front side) to the processing machine side becomessmaller than that before flexing of the mandrel 2 (see FIG. 3). To bespecific, the distance L between the contact surface 23 a and the worksurface 7 at the front side is smaller than that at the rear side beforeflexing of the mandrel 2 (see FIG. 3). With the mandrel 2 flexed (seeFIGS. 4 and 5), the front side of the outer peripheral surface 23 hasapproximately the same distance as that of the rear side thereof.Herein, the mandrel 2 before being flexed is a mandrel 2 without beingflexed, if the work surface 7 is assumed not to apply reaction force.

The diameter of the outer peripheral surface 23 on the mandrel 2 at theopposite side (front side) to the processing machine side is set to begreater than that at the processing machine side (rear side). Therefore,a parallel degree between the sectional line 27 and the sectional line72 with the mandrel 2 flexed during formation of the dimples 71 becomessmaller than that before flexing of the mandrel 2. The sectional line 27on the outer peripheral surface 23 is defined by sectioning the outerperipheral surface 23 by the plane M and is positioned on a side of theouter peripheral surface 23 closer to the work surface 7 (work surfaceside). The plane M includes the axis CL of the mandrel 2 and is normalto the work surface 7. The sectional line 72 on the work surface 7 isdefined by sectioning the work surface 7 by the plane M. The worksurface 7 is a conception that means a plane which is not only the worksurface 7 itself but also includes the work surface 7. To be specific,the sectional line 27 on the outer peripheral surface 23 at the worksurface 7 side and the sectional line 72 on the work surface 7 getcloser to each other toward the front side F1, not being parallel witheach other, while they are approximately parallel with each other withthe mandrel 2 being flexed (see FIG. 4). This parallel degree defines amagnitude of deviation (error) from a straight line which is to beparallel with the standard straight line.

This constitution is applied to the case in which the work surface 7 ofthe embodiment is a plane. With the mandrel 2 flexed during formation ofthe dimples 71, the sectional line 27 on the outer peripheral surface 23at the work surface 7 side and the sectional line 72 on the work surface7 are approximately parallel with each other, which enables all of theballs 3 arranged on the outer peripheral surface 23 to be pressedagainst the work surface 7 at a more uniform strength.

In the embodiment, the mandrel 2 has the front portion 25 having theouter peripheral surface 23 in a side-shape of a frustum of a circularcone which diverges in diameter toward the opposite side (front side)relative to the processing machine side. That is, the outer peripheralsurface 23 of the mandrel 2 is formed in a taper shape having a thickdistal end, which has a predetermined inclined angle (for example, 10minutes, or ⅙ degree).

According to this constitution, the sectional line 27 at the worksurface 7 side of the outer peripheral surface 23 with the mandrel 2flexed becomes strictly a curved line, not a straight line, the paralleldegree between the sectional line 27 at the work surface 7 side of theouter peripheral surface 23 and the sectional line 72 on the worksurface 7 is enabled to become sufficiently smaller. Formation of theouter peripheral surface 23 of the mandrel 2 in a side-shape of afrustum of a circular is made by minor modification from formation ofthe mandrel 2 in a side-shape of a straight circular cylinder in themanufacturing process of the mandrel 2 itself.

The mandrel 2, the balls 3 and the frame 4 are required to havedurability, are composed of a special alloy steel, and are heat-treatedfor enhancing hardness and toughness. They may be treated by surfacecoating such as DLC, TIN or TICN for enhancing durability, depending onworking condition. The mandrel 2 and the balls 3 may be composed of amaterial such as cemented carbide or a ceramic material.

Next, a dimple-forming burnishing method using the above-structuredburnishing tool 1 is described with reference to FIGS. 1 to 5 and FIGS.6 and 7.

FIGS. 6A and 6B are views which show formation of dimples by theburnishing tool. FIG. 6A is a general view as viewed from the positionopposed to the work surface. FIG. 6B is an enlarged view of the dimplesformed on the work surface as shown by FIG. 6A. FIGS. 7A, 7B and 7C areschematic views of the burnishing tool and the work surface when theburnishing tool is pressed into the work surface with the press-intoamount varied. FIG. 7A shows a case of a smaller press-into amount; FIG.7B shows a case of an appropriate press-into amount; and FIG. 7C shows acase of a larger press-into amount.

Firstly, an electric source (not shown in Figs) is started to permit theburnishing tool 1, fitted to a processing machine (not shown in Figs),for example, an NC milling machine, to rotate at a predeterminedrotational speed in a direction of R2 (see FIG. 2). At this time, theframe 4 is fixed to the mandrel 2, and the mandrel 2 and the frame 4rotate integrally with each other.

Subsequently, as shown in FIGS. 2 and 6, with the press-into amount forpressing the balls 3 into the work surface 7 set to a predeterminedvalue, the burnishing tool 1 is fed at a predetermined feed rate in adirection of the arrow Q, or an extension direction of the grove portionS.

As shown in FIG. 6, this operation permits the balls 3, held by theframe 4 on the outer peripheral surface 23 of the mandrel 2 rotating, tointermittently contact the work surface 7 corresponding to a sidesurface on the groove portion S of the work W, thereby forming dimples71 over a whole predetermined region of the work surface 7. The depthsof the dimples 71 become smaller than the press-into amounts of theballs 3 due to a repulsion force from the work W and the like.

The dimples 71 are formed on the work surface 7 at intervals of ahorizontal arrangement-pitch P1 in a peripheral direction of the mandrel2 or a direction of the arrow Q and at intervals of a verticalarrangement-pitch P2 in an axial direction parallel with the axis CL ofthe mandrel 2 (see FIG. 6B). The horizontal arrangement-pitch P1corresponds to a feed rate in a direction of the arrow Q per a rotationof the burnishing tool 1. The vertical arrangement-pitch P2 correspondsto each axial displacement between the balls 3 in each of the ballarrangements neighboring at intervals of 90 degree on the circularperiphery of the frame 4.

When forming the dimples 71 on the work surface 7 is finished, theburnishing tool 1 is separated apart from the work W to be stopped fromrotating.

Herein, the previous experiment determined said press-into amount bywhich the burnishing tool 1 is further pressed into the work surface 7from the position for the ball 3 at the most distal end side (frontside) to contact just and slightly the work surface 7.

As shown in FIG. 7A, if the press-into amount is too small, a flexion ofthe mandrel 2 becomes smaller. The balls 3, located at the front side onthe outer peripheral surface 23 diverging toward the distal end side(front side), are pressed against the work surface 7 more strongly thanthe balls 3 located at the rear side. Therefore, the dimples 3 formed atthe front side become deeper than the dimples 71 formed at the rearside. In the worst case, there is some possibility of not forming thedimples 71 at the rear side. As shown in FIG. 7C, if the press-intoamount is too large, flexion of the mandrel 2 becomes larger. The balls3 located at the rear side on the outer peripheral surface 23 arepressed against the work surface 7 more strongly than those located atthe front side. Therefore, the dimples 3 formed at the rear side becomedeeper than the dimples 71 formed at the front side. In the worst case,there is some possibility of not forming the dimples 71 at the frontside. On the other hand, as shown in FIG. 7B, if a press-into amount isappropriate, the sectional line 27 on the work surface 7 side of theouter peripheral surface 23 and the sectional line 72 on the worksurface 7 become approximately parallel with each other with the mandrel2 flexed. The balls 3 located at the front side of the outer peripheralsurface 23 and the balls 3 located at the rear side are pressed againstthe work surface 7 at a further uniform strength. This operation permitsthe depths of the dimples 71 formed at the front side and the depths ofthe dimples 71 at the rear side to become approximately uniform.

Therefore, as shown in FIG. 7B, the experiment determined the press-intoamount for forming the dimples 71 of approximately a uniform depth onthe work surface 7.

A size, a depth, an arrangement-pitch and an arrangement aspect of thedimples 71 may be modified by varying a machining condition (a rotationrate of the burnishing tool 1 and a feed rate); a diameter, anarrangement layout, the number, the arrangement layout and the mass ofthe balls 3; an inclined angle (taper angle) of the outer peripheralsurface 23 on the mandrel 2; a force of pressing the balls 3 against thework surface 7 and the like. Following this, a material property(hardness) of the work surface 7 on the work W may be considered.

The burnishing tool 1 is structured so as to be applied to formation ofthe dimples 71 on the work surface 7 corresponding to the side surfaceof the groove portion S that has a depth of about 20 mm to about 50 mmand a width of about 20 mm. If the burnishing tool 1 is applied toanother groove portion S of a different size and the like, the measuremay be taken by fabricating a burnishing tool 1 of a different size indiameter and length at the design stage or by removing the frame 4 andthe balls 3 from the mandrel 2 to be exchanged to ones of a differentspecification.

As describe above, the burnishing tool 1 of the embodiment includes: themandrel 2 having a rear end side fitted to the processing machine forrotating; and the frame 4 fitted and fixed to the front end side of themandrel 2 from the outside to hold the balls 3 that contact the outerperipheral surface 23 on the mandrel 2. With the mandrel 2 and the frame4 rotating, the balls 3 on the outer peripheral surface 23 contact withthe work surface 7, thereby forming the dimples 71 on the work surface7. Herein, the diameter of the outer peripheral on the mandrel 2 is setin such a way that in relation to the distance L between the contactsurface 23 a with the balls 3 on the work surface 7 side of the outerperipheral surface 23 and the work surface 7 and normal to the worksurface 7, the distance difference between the front side and the rearside with the mandrel 2 flexed during formation of the dimples 71becomes smaller than that before flexing of the mandrel 2.

According to this embodiment, if the mandrel 2 is flexed duringformation of the dimples 71 due to the burnishing tool 1 of a smallerdiameter, all of the balls 3 arranged on the outer peripheral surface 23are pressed against the work surface 7 at a further uniform strengthwith the mandrel 2 being flexed. This operation forms the dimples 71 ofapproximately a uniform depth, for example, on the whole predeterminedregion on the work surface 7 of a narrow portion such as a side surfaceof the groove portion S. That is, the dimples 71 are enabled to beefficiently and securely formed on the work surface 7 of the narrowportion such as the side surface of the groove portion S.

Therefore, the work surface 7 on the work W is formed with very fine anduniform dimples 71, which are capable of keeping lubricant. Thisfunction prevents oil from running out on the work surface 7, andreduces frictional resistance, thereby enhancing the sliding property.

The burnishing tool 1 is fitted to a generic processing machine (amachine tool) such as an NC milling machine, and is capable of formingdimples in a shape which is required at ease and an inexpensive price.

As described above, though the embodiment of the invention is explained,the invention is not limited to the constitution as described in theembodiment. Although the invention has been described above by referenceto the embodiment, the invention is not limited to the embodimentsdescribed above. Appropriate modifications and variations of theconstitution may be made within the purpose of the invention, includingappropriate combination and selection of the constitutions as describedin the embodiment.

For example, the embodiment explains that the burnishing tool 1 isfitted to a processing machine of a vertical type with the axis CLextending in a vertical direction. However, the invention is not limitedto this aspect, and, for example, the burnishing tool 1 may be fitted toa machining tool of a horizontal type with the axis CL extending in ahorizontal direction for use.

Although the above-described embodiment explains that the work surface 7is a plane parallel with a vertical axis, the present invention is notlimited to this aspect. For example, the present invention is applicableto the case where work surface 7 is curved in a section which is definedby sectioning the work surface 7 by a horizontal plane (planar view). Tobe specific, the invention is applicable to the case where the worksurface 7 is a side surface of a scroll groove in a compressor componentor the case where the work surface 7 is an inner peripheral surface 7 ofa hole formed to the work W and the like. The invention is applicable tothe case where the work surface 7 is a plane inclined to the verticalaxis.

In this case, the burnishing tool 1 is fitted to, for example, the fiveaxis processing machine, and is used with the axis CL being parallelwith the work surface 7. That is, considering a shape and an angle ofthe work surface 7, a movement control and a rotation (rotary motion)control are combined and performed to form the dimples 71 on the worksurface 7 of the work W. The movement control is a control of moving inthree axis directions, the X axis, the Y axis and the Z axis orthogonalto each other, of a machining head (not show in Figs) for fitting theburnishing tool 1. The rotation control is a control of rotating abouttwo axis, or the A/B axis and the C axis, of a work-fixing portion (notshown in Figs) for fixing the work W.

In addition, the present invention is applicable to the case where thework surface 7 is curved in a section (as viewed from the front) whichis defined by sectioning the work surface 7 by, for example, thevertical plane. In this case, with the mandrel 2 flexed during theformation of the dimples 71, the diameter of the outer peripheralsurface 23 a varies along the axis CL, depending on a shape of thecurved work surface 7, and is set in such a way that the distance Lbetween the contact surface 23 a and the work surface 7 at the frontside of the outer peripheral surface on the mandrel 2 becomesapproximately the same as that at the rear side thereof.

In the above-described embodiment, although a ball 3 is fitted to acircular through-hole formed to the frame 4, the present invention isnot limited to this. The frame 4 may be formed with elongated circularthrough-holes, to which a plurality of balls 3 are fitted.

EXPLANATION OF REFERENCE NUMERALS

-   1 dimple-forming burnishing tool-   2 mandrel-   23 outer peripheral surface-   23 a contact surface-   27 cutting-plane line-   3 ball-   4 frame-   41 frame body-   42 frame cover-   43 first through-hole-   48 second through-hole-   7 work surface-   71 dimple-   72 cutting-plane line-   CL axis-   L distance-   M plane

What is claimed is:
 1. A dimple-forming burnishing tool comprising: amandrel having an end side fitted to a processing machine for rotating;and a frame fitted and fixed to another side of the mandrel from theoutside, and holding balls contacting an outer peripheral surface of themandrel, wherein the balls on the outer peripheral surface contact awork surface for forming dimples on the work surface with the mandreland the frame rotated, wherein the outer peripheral surface has acontact surface with the balls and, wherein the contact surface is at aside of the outer peripheral surface which is closer to the worksurface, wherein the contact surface and the work surface have adistance therebetween in a direction normal to the work surface, whereina distance at a side of the mandrel closer to the processing machine anda distance at the opposite side of the mandrel have a distancedifference therebetween, wherein a diameter of the outer peripheralsurface on the mandrel is set in such a way that the distance differencewith the mandrel flexed during formation of the dimples becomes smallerthan that before flexing of the mandrel.
 2. The dimple-formingburnishing tool according to claim 1, wherein the mandrel has a firstsectional line which is positioned on the outer peripheral surface andat said side closer to the work surface, wherein the first sectionalline is defined by sectioning the mandrel by a plane which is normal tothe work surface and includes an axis of the mandrel, wherein the worksurface has a second sectional line on the work surface, wherein thesecond sectional line is defined by sectioning the work surface by theplane, wherein the first sectional line and the second sectional linehave a parallel degree therebetween, wherein a diameter of the outerperipheral surface on the mandrel at the opposite side to said sidecloser to the processing machine is set to be larger than that at theside closer to the processing machine, so that the parallel degree withthe mandrel flexed during formation of the dimples becomes smaller thanthat before flexing of the mandrel.
 3. The dimple-forming burnishingtool according to claim 2, wherein the outer peripheral surface of themandrel has a side shape in a frustum of a circular cone which divergestoward the opposite side to said side closer to the processing machine.4. The dimple-forming burnishing tool according to claim 1, wherein theballs are spaced apart from each other in a peripheral direction of theframe and in a direction parallel with the axis of the frame.
 5. Thedimple-forming burnishing tool according to claim 1, wherein the framecomprising: a frame body fitted to the mandrel from the outside anddefining first through-holes for the balls to be fitted thereto,respectively; and a frame cover fitted to the frame body from theoutside, having the balls projecting radially outward from the outerperipheral surface of the frame, and defining second through-holes whichcoincide in position with the first through-holes for preventing theballs from dropping out of the first through-holes, respectively.
 6. Adimple-forming burnishing method comprising: contacting the balls on thework surface to form dimples on the work surface using thedimple-forming burnishing tool according to claim 1.